VERTEBRATA 147 



caudal may be homocercal, as in ordinary species ; or 

 heterocercal, as in sharks. In ancient heterocercal 

 fishes, the tail was frequently vertebrated. The pectoral 

 and ventral fins stand for the fore and hind limbs of 

 other vertebrates. As the specific gravity of the body 

 is greater than that of the water, most fishes are pro- 

 vided with an air bladder, which is an outgrowth from 

 the esophagus. This is absent in such as grovel at the 

 bottom, as the rays, and in those, like the sharks, en- 

 dowed with compensating muscular power. 



Fishes have no prehensile organ besides the mouth. 

 Both jaws are movable. The teeth are numerous, and 



FIG. 121. Salmon (Salmo salar). Both hemispheres. 



may be recurved spines, as in the pike ; flat and triangu- 

 lar, with serrated edges, in the shark ; or flat and tessel- 

 lated in the ray (Fig. 230). They feed principally on 

 animal matter. The digestive tract is relatively shorter 

 than in other vertebrates. The blood is red, and the 

 heart has rarely more than two cavities, an auricle and 

 a ventricle, both on the venous side. Ordinary fishes 

 have four gills, which are covered by the operculum, and 

 the water escapes from an opening behind this. In 

 sharks there is no operculum, and each gill pouch opens 

 separately. The brain consists of several ganglia placed 

 one behind the other, and occupies but a small part of 

 the cranial cavity (Fig. 336). Its average weight to the 



